Fiber amplifiers provide gain by the stimulated emission of radiation. A fiber amplifier typically includes a gain fiber, the core of which includes active dopant ions. A signal to be amplified as well as a pump signal are provided to the fiber core, and a wavelength division multiplexer (WDM) coupler can be used for this purpose. The pump signal is typically generated by a laser diode.
The amplifier gain is related to the amount of pump power coupled to the gain fiber. Also, the output power of the amplifier influences the frequency response of the amplifier to signal modulation. In particular, as the output power increases, the amplifier becomes less effective for low frequency components of the input signal. The amplifier has a high pass frequency response, which shifts towards higher frequencies for increased output power. The low frequency response is dependent on the time constant of the amplifier, which is influenced by the intrinsic time constant of the erbium doped fiber (approximately 10 ms), the effective area of the fiber and the power levels.
There are, however, increasing power demands on optical amplifiers for use in WDM (wavelength division multiplex) optical communications systems. In these systems, a number of optical channels are located in close proximity across an operating bandwidth of the system, for example covering wavelengths of 1530 to 1610 nm. As the number of channels increases, the total output power requirements of the amplifier increase, and it is more difficult to keep cross talk between channels to acceptable levels.
Good overlap between the pump and signal field distributions and the erbium doping in the core is desirable. This can be accomplished by providing the gain fiber with a relatively small mode field diameter (MFD), a characteristic that causes the optical power to be concentrated in a relatively small area along the fiber axis. This increases the field intensity for fixed output powers and improves performance, particularly at low pump powers. A “high gain” or “high efficiency” fiber can be achieved by employing a relatively large refractive index difference between the core and cladding and a relatively small core diameter. Typically, the MFD for high efficiency fibers is less than the MFD of standard telecommunication fibers, by a factor of at least 1.5:1. Conventional telecommunication fibers typically have mode field diameters in the range of 9 μm to 11 μm for light at 1550 nm.
Amplifier fibers are typically designed to ensure single mode operation of the fiber at the signal and pump wavelengths. Typically, the pump signals have a wavelength of 980 nm and/or in the range 1450-1500 nm, and the fiber is therefore designed to have a single mode cut-off wavelength below 980 nm, so that for all wavelengths at or above 980 nm, the fiber operates as a single mode waveguide. Low bending losses are desired at the longest signal wavelength, and the requirement for single mode operation below 980 nm (for example at 97 (nm) and good bend performance at long wavelengths forces the use of small mode field diameter fiber with a relatively large refractive index difference. The bend performance of the fiber is improved by increasing the index difference between the core and cladding, while reducing the core diameter to maintain the cut-off wavelength at the required value.
One problem with the use of an amplifier using this conventional type of doped fiber, which occurs particularly at high operating powers, is the corruption of low frequency signals. This is aggravated by the low mode field diameter of conventional amplifier fiber, which tends to increase the low frequency attenuation. Systems using optical signals in the SONET or SDH format have relatively low frequency components. Some implementations of optical communications systems also use a low frequency analogue maintenance channel. This is a low modulation depth amplitude modulated signal which is superimposed over the signal data. This maintenance channel has a low data rate than the signal and can therefore be read using low speed electro-optic circuitry. However, the low data rate of this maintenance channel makes it vulnerable to the poor low frequency response of conventional amplifiers at high operating powers.